Familial X centromere variant resulting in false-positive prenatal diagnosis of monosomy X by interphase FISH

Familial 18 centromere variant resulting in difficulties in interpreting prenatal interphase FISH

We report here on a familial case of centromeric heteromorphism of chromosome 18 detected by prenatal interphase fluorescence in situ hybridization (FISH) analysis transmitted by the mother to her fetus, and resulting in complete loss of one 18 signal. The prenatal diagnosis was performed by interphase FISH (AneuVysion probe set, and LSI DiGeorge 22q11.2 kit) because of the presence of an isolated fetal cardiac abnormality, and was first difficult to interpret: only one centromeric 18 signal was detectable on prenatal interphase nuclei, along with one signal for the Y and one for the X chromosome. The LSI DiGeorge 22q11.2 kit also showed the absence of one TUPLE 1 signal on all examined nuclei. In fact, the FISH performed on maternal buccal smear displayed the same absence of one chromosome 18 centromeric signal, combined with the presence of two TUPLE1 signals. All these results led to the diagnosis of an isolated 22q11.2 fetal microdeletion that was confirmed on metaphases spreads. This case illustrates once again that the locus specific (LSI) probes are more effective than the alpha centromeric probes for interphase analysis. The development of high-quality LSI probes for chromosomes 18, X and Y could avoid the misinterpretation of prenatal interphase FISH leading to numerous additional and expensive investigations.

Interphase M-FISH applications using commercial probes in prenatal and PGD diagnostics

Early, rapid and reliable diagnosis is of first priority in prenatal medicine. The combination of specific sonographic markers (e.g. nuchal translucency) and biochemical parameters in maternal serum (e.g. free beta-human chorionic gonadotropin, pregnancy-associated plasma protein A), has already dramatically improved the sensitivity of non-invasive first trimester risk screening in pregnancy. In invasive prenatal diagnosis, in addition to well-established chorionic villi short-term culture, interphase multi-colour-fluorescence in situ hybridisation (M-FISH) on uncultured amnion cells has become a reliable tool for the rapid detection of fetal aneuploidies. Interphase M-FISH applications have enabled the diagnosis of selected chromosomal abnormalities in single cells and, therefore, have also become an important diagnostic tool for preimplantation diagnosis (PGD). The development of commercially available probe sets, in particular, has led to a broad use of interphase M-FISH in prenatal and PGD diagnosis.

French multi-centric study of 2000 amniotic fluid interphase FISH analyses from high-risk pregnancies and review of the literature

This prospective and multi-centric study confirms the accuracy and the limitations of interphase FISH and shows that any cytogenetics laboratory can perform this technique. With regard to the technical approach, we think that slides must be examined by two investigators, because the scoring may be subjective. The main problem with the AneuVysion kit concerns the alpha satellite probes, and especially the chromosome 18 probe, which is sometimes very difficult to interpret because of the high variability of the size of the spots, and this may lead to false negative and uninformative cases. The best solution would be to replace these probes by locus-specific probes. Concerning clinical management, we offer interphase FISH only in very high-risk pregnancies or/and at late gestational age because of the cost of the test. We think that an aberrant FISH result can be used for a clinical decision when it is associated with a corresponding abnormal ultrasound scan. In other cases, most of the time, we prefer to wait for the standard karyotype.